The high oil prices now prevailing have increased the interest in the great supplies of coal. Different methods have been suggested to make use of these supplies. One method proposed for the transportation and burning or gasifying of coal involves pulverizing the coal into very fine particles and, optionally, purifying the coal. Then the particles are dispersed into water to a slurry that can be transported to industrial and heating plants for use in the chemical and energy field.
The above mentioned method to transport and burn or gasify coal is e.g. described in the U.S. Pat. Nos. 3,524,682 and 4,217,109 and in the European patent application No. 8628.
The U.S. Pat. No. 3,524,682 discloses that it is possible to facilitate the pumping of suspensions of pulverized solid materials, such as minerals and coal, by adding a small quantity of a water soluble polyelectrolyte, obtained by polymerization of acrylic acid, acrylonitrile and similar compounds, to a system of water and pulverized solid material, the molecular weights of the polyelectrolytes preferably varying between 100 000 and 3.times.10.sup.6.
Furthermore, the U.S. Pat. No. 4,217,109 discloses that polyelectrolytes of e.g. polycarboxylic acids or salts thereof, e.g. polyacrylates, can be used as additives in suspensions of coal and water. According to this patent the polyelectrolyte acts i.a. as an agent for separating the coal from other substances in the coal/water-slurry.
In both these U.S. patents it is stated that it is suitable to work with a water content of about 40% by weight (see Example 1 in the U.S. Pat. No. 3,524,682 and column 4 lines 31-33 in the U.S. Pat. No. 4,217,109).
In the European patent application No. 8628 there is disclosed a slurry of coal in water, also containing a water soluble polymer that might be poly(ethylene oxide), poly(acrylamide), hydroxyethyl cellulose, quaternary nitrogen-substituted cellulose esthers, xanthan gum, hydroxy propyl guar gum and carboxymethyl hydroxypropyl guar gum. This patent application states that it is possible to produce transportable coal/water slurries with a coal content of up to 78% by weight. However, according to the patent application, these slurries with an extremely high percentage of coal are not transported by normal pumps but by extruders, intended for materials with very high viscosity. The running of these extruders requires a great amount of energy and therefore it is absolutely unrealistic to use them for long-distance transports.
A big problem with these known methods for transporting coal is to achieve pumpability at low water contents, i.e. high percentages of coal in the dispersions. A high content of coal is desirable to get good transport efficiency for the coal. According to our experience the addition of the above mentioned polymers to dispersions of coal in water gives mixtures with too high viscosity to be transported at reasonable costs.